This invention relates to amplifier circuitry and in particular to DC amplifiers exhibiting low-level voltage offset drift. Amplifiers of this type find application in generation stable reference potentials.
In the design of precision circuitry such as analog-to-digital (A/D) converters for example it is necessary to generate stable reference potentials against which the signal to be measured may be compared. The accuracy of such a reference for a 4000 count A/D must be in the order of 1 part in 10,000 and it must be relatively stable over a reasonable temperature range and during the lifetime of the device. Such references frequently employ differential amplifiers to measure potentials across like resistors indicative of the current conducted therethrough, the amplifier output potential being used in a feedback loop to readjust the currents for temperature induced changes. See for example U.S. Pat. No. 4,263,519 entitled "Bandgap Reference" (incorporated herein by reference). In order for the output potential of the reference to be stable, the amplifier must be substantially stable i.e. free of parameter drift which is reflected as a change in input offset.
Dingwall et al. (U.S. Pat. No. 4,068,182 entitled "Direct Coupled Cascade Amplifier With Automatically Adjusted Quiescent Output Signal Level" issued Jan. 10, 1978) designed an amplifier which is unaffected by elemental parameter drift by providing for periodic autozeroing of the amplifier. During autozero the amplifier input terminals are shorted, the concurrent consequent output being a measure of present amplifier offset. This offset is fed back internally, stored on a capacitor, and used to adjust amplifier gain until the next autozero interval. The shortcoming of the Dingwall et al arrangement is that the amplifier is effectively out of useful service during the autozero intervals. This becomes a relatively large percentage of the total time where a highly stable device is required.
Shaffer in U.S. Pat. No. 4,138,649 issued Feb. 6, 1979 entitled "Amplifier Systems" set forth a solution to amplifier offset in DC amplifiers which provides essentially for 100 percent duty cycle. This amplifier includes switches at the input and output stages for alternating the application of the input signal between the inverting and non-inverting input terminals and simultaneously interchanging the output signal between one of the two available differential output connections. Switching the signals is intended to average out the first stage equivalent offset signal. The Shaffer circuit only compensates for offsets in the amplifier differential input pair of devices i.e. the input emitter coupled (bipolar) or input source coupled (FET) long tail pair. Where imperfectly matched load devices are employed in the collector or drain connections of the input devices, further input-offset errors will be introduced. These load related errors not only limit the effectiveness of the offset correction process but in addition, it has been found that such load related offsets do not in fact generally remain constant. Consider, for example, that the active load elements generate an offset of 5 mV. In addition to this error if the offset drifts by only 5 percent a 0.25 mV shift will be generated. Consider, also, that it is desired to produce in a reference potential circuit a reference potential of 1 Volt stable to within 1 part in 10,000 or 0.1 mV. It is readily seen that a 0.25 mV shift is intolerable.
To eliminate amplifier offset and drift due to the active load elements of a differential amplifier, the Shaffer type amplifier has been improved by providing means to sequentially permutate the particular active load elements between the particular input transistors synchronously with the permutation or switching of the amplifier input and output connections. The 1/f or flicker noise generated at frequencies below the switching frequency is a time-varying parameter which is also averaged out by the process. The amplifier therefore also exhibits a lower flicker noise component in its output signal.